Problem: $\dfrac{5}{8} - \dfrac{1}{5} = {?}$
Answer: $ = {\dfrac{5 \times 5}{8 \times 5}} - {\dfrac{1 \times 8}{5 \times 8}} $ $ = {\dfrac{25}{40}} - {\dfrac{8}{40}} $ $ = \dfrac{{25} - {8}}{40} $ $ = \dfrac{17}{40}$